HEWLETT- PACKARD JOURNAL T E C H N I C A L I N F O R M A T I O N F R O M T H E - h p - VOL. 1 No. 10 L A B O R A T O R I E S JLISHED BY THE HEWLETT-PACKARD COMPANY, 395 PAGE MILL ROAD, PALO ALTO, CALIFORNIA JUNE, 1950 The Explanation for Certain Cathode-Follower Effects ONE of the problems when using a highimpedance voltmeter to measure volt age in high-impedance audio and video-fre quency circuits is minimiz ing the effect of test lead ca pacity. Usually, shielded test leads are necessary to prevent hum pick-up, but the capacity of such leads is likely to load seriously the circuit under test. For example, a set of shielded test leads may add 1 00 micro- Figure 1. Model 45 1 A Bridging Amplifier bus 83-megobm positive input impedance. PRINTED IN U.S. micro-farads to the circuit under test. This represents a reactive loading at 10 kc of 160,000 ohmsâ€” a very appreciable loading across a one-half or one megohm circuit. A review of the literature pertaining to this problem shows that the usually-recom mended panacea is the use of a cathode-fol lower input with a shielded cable connected between grid and cathode. This connection divides the input capacity roughly by the feedback factor of the cathode follower. Re sistance-wise, the grid resistor can be con nected so that its value will be increased by the same feedback factor, making obtainable an input impedance as high as 100 megohms. At first, this arrangement seems an almost ideal solution to the problem, but examina tion of the circuit discloses major shortcom ings. Consider Figure 2 which shows the cathode-follower used as an impedancetransformer between a high-impedance stage and a measuring instrument. A doubleshielded cable is used in the input circuit to minimize hum pick-up. The output circuit is shown to include the capacity of the test lead to the measuring equipment as well as the input capacity of the measuring equipment itself. A C O P Y R I G H T © Copr. 1949-1998 Hewlett-Packard Co. I 9 5 O H E W L E T T - P A C K A R D C O . Figure 2. (a) Cathode follower used as impedance transformer, (b) Same circuit u'itb capacities lumped. A mathematical investigation shows that the circuit of Figure 2(b) has the approximate equivalent cir cuit shown in Figure 3(a). This equivalent circuit includes negative capacity components as well as a negative resistance component. In other words the impedance-trans forming cathode-follower amplifier of Figure 2 is itself a voltage gener ator and will feed energy back into the circuit being measured under certain conditions. Evaluating the circuit of Figure 3(a), it will be seen that at very low frequencies the resistances Rj and R> normally become insignificant com pared to the reactances of the capaci tors in the upper portion of the branch. This therefore leads to the equivalent circuit shown in Figure 3(b). Here the negative capacities â€” Cs and â€” Ck are exactly compen sated by the positive capacities CÂ» and Ck. Hence, the input circuit of the cathode follower consists of (a) the grid resistance multiplied by the feedback factor of the cathode fol lower (b) in parallel with the grid capacity divided by the feedback fac tor (Figure 3(c)â€” all according to conventional theory. However, as frequency increases and the reactance of the negative ca pacities becomes less, the negative resistance component becomes more influential. The effect of the cathode follower's negative impedance can often be observed by connecting the circuit of Figure 2 across an induc tance, for all the conditions neces sary for oscillation are then present and the circuit will frequently oscil late for a wide range of inductance values. In a voltage- measuring appli cation, this oscillation will com pletely prevent any real measure ment of voltage. If the circuit is con nected across other than an induc tance, the action of the cathode fol lower may increase the voltage at the point of measurement, also giv ing inaccurate results. Another facet of the impedance of cathode followers involves the load that is driven by the cathode circuit. If the input impedance of the cath ode follower is to be made reason ably high, a relatively high value of cathode resistance is desirable. If, at the same time, the low output im pedance of the cathode follower is to be used to drive a low-impedance load, the effective value of the cath ode resistor is reduced. This reduc tion may easily be as much as two to one. The lowered effective value of Figure 3- (a) Equivalent circuit of Figure 2. (b) and (c) Low frequency equivalent circuit. © Copr. 1949-1998 Hewlett-Packard Co. J i - ^ / V ^ R, Figure 4. Resistance-capacity voltage divider. cathode resistance will then be re flected in a reduction in input im pedance by this same factor, for the input impedance is proportional to (l+G,,,Rk). In other words the cathode follower can be used to ob tain a high input impedance or a low output impedance, but full advan tage cannot be taken of both charac teristics at the same time. For the above reasons the cathode follower is not considered to be a satisfactory solution to the original problem, which was to reduce the effect of the capacity of the test leads in combination with the capacity of the measuring equipment. Another approach that eliminates the cathode follower and still achieves this ob jective is the simple resistance-ca pacity voltage divider shown in Fig ure 4. In Figure 4 if the ratio of the ca pacities equals the ratio of the re sistances (CP/C=RS/R,,), the volt age divider action is constant at all frequencies (E0/Ei=R1,/(Rp+Rs)). To solve the original problem, the divider of Figure 4 can be designed to use the capacity of a test cable as part of the voltage divider, as in Fig ure 5. Here, the capacity Cp is the ca pacity of the shielded cable and has been absorbed into the circuit so that it is useful. By properly choosing R, and the length and type of cable, and by taking care to avoid excess capa city in the shield around Rs, a test lead with an input impedance of low capacity and high resistance is ob- SHIELDED PROBE Figure 5. R-C divider incorporated into high-2 probe. tainable. Thus, it is possible to make the input resistance RS+R,, well over 50 megohms and to keep the total input capacitance within approxi mately 5 mmf. To compensate for the loss in volt age caused by the voltage divider action, a good amplifier whose gain is stabilized by feedback at a value inversely related to the reduction in the voltage divider (E0/EÂ¡) can be connected across the output of the divider. We then have a one-to-one circuit that can be connected be tween the test point and any of the usual pieces of measuring equip ment ordinarily on hand. This ar rangement is the substance of the -hp- Model 451 A Bridging Ampli fier. With about a 15:1 reduction in the voltage divider, this circuit can only exhibit a fixed positive input resistance. If the amplifier is prop erly designed, it introduces negli gible error into the system. Further, the output impedance of the ampli fier can be made even lower than that of the cathode follower. Owing to the isolation by the input divider, any excessive loading of the ampli fier output can not affect the circuit under test. The -hp- 451 A is designed in the above manner and provides the de sirable features of a shielded input cable with a low-capacity, high-re sistance input and a low-impedance output system, all at moderate cost. Any of the usual pieces of test equip ment can then be connected to its output. The input capacity is ap proximately 5 mmf and the input re sistance in excess of 83 megohms over the range from 20 cps to 200 kc. Distortion produced in the amplifier is less than 0.1 % up to 30 volts out put. To permit measurement of volt ages higher than 30, a switch in the unit increases the loss in the input divider circuit by a factor of 10, al lowing voltages up to 300 to be measured at an overall gain of onetenth. The frequency response is flat within 2% from 20 cps to 200 kc and within 1 db from 20 cps to 1 me. Hum and noise across the output terminals are less than 4 millivolts. Two sets of output terminals permit monitoring of the test voltage by two different measuring instru ments. â€”Brtmton Bauer. SPECIFICATIONS FOR -hpMODEL 451A BRIDGING AMPLIFIER FREQUENCY RESPONSE: Constant within 2% from 20 cps to 200 kc; constant within 1 db from 20 cps to 1 me. INPUT IMPEDANCE: Approximately 83 meg ohms shunted by 5 mmf. INPUT VOITAGE RANGE: Up to 30 volts at gain of unity; up to 300 volts at gain of one-tenth. MAXIMUM OUTPUT VOLTAGE: 30 volts. DISTORTION: less than 0.1% from 20 cps to 200 kc when loaded with 0.1 megohm or more. OUTPUT NOISE: Less than .004 volts includ ing hum. OUTPUT IMPEDANCE: Less than 5 ohms above 5 kc. POWER SUPPLY: Operates from nominal 115 v, 50/60 cycle source; requires 40 watts. CABIES SUPPLIED: Input probe and cable permanently attached; 7' power cable permanently attached. DIMENSIONS: 8</2" wide, 5Vl" high, 11" deep. WEIGHT: Approx. 9 fbs.; shipping weight, 22 Ibs. PRÃCE: $100 f.o.b. Palo Alto, California. Data subject to change without notice. Extended -Range Metered Power Supply The popular -hp- Model 710A reg ulated power supply was designed as a low-power source and has proved very useful for general laboratory applications. Now, in accordance with requests for a regulated supply with higher voltage and current ca pacity, the new -hp- Model 712A has been designed. This economical supply is useable in applications where regulated dc voltages up to 500 volts at currents up to 200 milli- amperes are required. Both the out put voltage and current are directly indicated by large 4-inch panel meters. A regulated 0- 1 50 volt nega tive dc adjustable bias supply and ac filament power are also provided. Current overload fuses are located in convenient panel mounting posts. The high- voltage supply is adjust able from 500 volts down to less than 1 volt in one continuous sweep and regulates within 0.5 rc or 0.1 volt, © Copr. 1949-1998 Hewlett-Packard Co. whichever is larger, from no-load to full-load. The negative terminal of the high-voltage supply is connected internally to the positive terminal of the bias supply so that the bias sup ply is always negative with respect to the negative terminal of the highvoltage supply. However, either the positive or negative terminal of the high-voltage supply can be ground ed. The panel voltmeter that moni tors the high-voltage has two ranges, -hp- voltages 7 12 A Regulated Power Supply provides regulated voltages adjustable from 0-500 rolls at currents up to 200 ma. 0-150 vdc and 0-500 vdc, to permit greater accuracy in setting output voltages. The current from the highvoltage supply is read on a 0-200 milliampere meter. The bias supply is adjustable up to â€” 150 volts dc and will provide cur rents up to 5 ma. At maximum volt age, regulation is within 1' "< from no-load to full-load. In the worst condition, the internal impedance of the bias supply is approximately 6500 ohms. However, the 0-150 volt range of the panel voltmeter can be switched to the bias supply to permit accurate adjustment of bias voltage. The filament supply provides 6.3 volts ac at currents up to 10 amperes maximum. Regulation of the fila ment supply is not provided. A cen ter-tap on the transformer winding for the filament supply is brought out to the front panel to be ground ed when desired. The filament wind ing can be operated at dc voltages up to approximately 500 volts, depend ing upon the external circuit. The -hp- 712A has been designed for reliability and long trouble-free performance. Oil-filled paper capa citors are used instead of electrolytics. By means of a special circuit ar rangement, the control tubes in the circuit are operated at only about SPECIFICATIONS FOR MODEL 712A REGULATED POWER SUPPLY OUTPUT VOLTAGE No. I: Regulated; 0-500 volts dc without switching; 200 ma maxi mum load. OUTPUT VOLTAGE No. 2. Regulated; 0-150 volts dc negative; 5 ma maximum load. OUTPUT VOLTAGE No. 3: Unregulated; 6.3 volts ac center-tapped; 10 amperes maxi mum load. REGULATION: HV No. 1, within 0.5% or 0.1 volt, whichever is larger, from no load to full load at any line voltage from 105 to 125 volts. HV No. 2, within 1% from no load to full load at maximum output volt age. Regulation at other voltages depends on setting of voltage control; internal impedance may be as high as 6500 ohms. METERS: Current, 0-200 ma. Voltmeter, 2 ranges, 0-150 and 0-500 volts. 0-150 volt range can be switched to read HV No. 2 output voltage. © Copr. 1949-1998 Hewlett-Packard Co. 75' r of rated plate dissipation, whereas, in this type circuit the plate dissipation of the control tube usu ally is a limiting factor. The above features combined with the moder ate price of the -hp- 712A make it well adapted to general-purpose work. HUM: Less than 8 millivolts. TERMINALS: Either positive or negative ter minal of HV No. 1 can be grounded. Posi tive terminal of HV No. 2 is internally connected to negative terminal of HV No. 1. INPUT POWER: Operates from 105-125 volt, 50 60 cycle supply; requires approxi mately 400 watts. OVERLOAD PROTECT/ON: Load and line sep arately fused. Fuses available on front panel. MOUNTING: Relay rack mounting; end frames are available to convert to table mounting but are not essential. SIZE: 10'/2" high, 19" wide, 13" deep. WEIGHT- 60 Ibs. Shipping weight, approx. 110 Ibs. PRICE: 5250.00 f.o.b. Palo Alto, Calif. End frames $5.00 per pair f.o.b. Palo Alto, Calif. (Specify No. 17.) Data subject to change without notice.